| Safety accidents of new energy vehicles increase extensively accompany with the continuously increasing of production and consumption.But it is hard to locate the cause of the accidents in batteries and describe the propagation process in detail limited by the current testing technology.Nowadays an increasing amount of high specific energy batteries are coming into service.Actually,the traction batteries are not able to be optimized with the high energy density and high safety for both simultaneously with existing technology.Therefore,in order to promise vehicles safety,it is necessary to carry out innovative research on test and evaluation technology for traction battery.On the one hand,the performance of traction battery and its change law should be accurately measured to support the formulation and dynamic adjustment of vehicle control strategy,and also to support the recycle and reuse of traction battery after retirement on the other hand.This paper takes the NCM811/Si O traction battery as object to carry out the research on life and safety evaluation technology.Besides,the evaluation test is carried out to verify the reutilization process of retired traction batteries.The details are as follows:The thesis studies the relationship between battery capacity,internal resistance and thermal characteristics and battery state of health under normal temperature cycle,high temperature cycle and working condition cycle,and the experimental data are mathematically fitted and processed to determine the method of estimating battery cycle life and state of health.The results show that:（1）The 0.5C standard cycle life curve of high nickel/silicon carbon battery at room temperature can be fitted into two stages with different decay trends.Stage I is a power function,stage II is a linear decay,the split point corresponds to the capacity retention rate of the battery is about70%.The point With the help of non-destructive tomography and ultrasonic scanning,it is found that the reason for the rapid decline of battery capacity in the later stage is that the internal structure of the sample changes significantly and the rate of change is accelerated.（2）Under high temperature conditions,the 0.5C standard cycle life curve can be fitted to a quadratic polynomial.（3）Based on the comparative analysis of the normal temperature standard cycle and the working condition cycle,it is determined that the capacity retention rate decay rate slope-（3±1）×10-4is the sign of rapid decay,which verifies that the DC internal resistance can be used as an evaluation index for the state of health.And a method is proposed to obtain the charging DC internal resistance by means of pulse current in the charging process to evaluate and predict the health state of electric vehicle batteries.This research work lays a good research foundation for further accurate prediction of battery cycle life in actual use and evaluation of battery health of electric vehicles.Aiming at the thermal safety of high specific energy traction batteries,the thesis uses adiabatic calorimeter and thermogravimetric analysis to carry out thermal runaway tests of high nickel/silicon carbon batteries and thermal characteristics of related materials,and studies these effects on thermal stability of batteries from the state of charge and state of health.The results show that:（1）Among the three materials of cathode,anode and separator,the thermal stability of the separator is relatively poor,and it will be destroyed at about 149℃,which may lead to internal short circuit in cathode and anode;（2）The thermal stability of the battery decreases as the state of charge of the battery increases,and the self-generated heat temperature of the battery thermal runaway decreases from 123℃in the zero state of charge to about 95℃in the 100%state of charge;（3）The thermal stability of the battery decays with the state of health of the battery with a trend of first increasing and then decreasing,the self-generated heat temperature of the battery thermal runaway increases from 71°C in the 100%healthy state to 82℃in the 80%healthy state,and then gradually decreases to about 62℃in the 40%healthy state.The results of this research work lay a solid foundation for the formulation and optimization of thermal management and thermal protection strategies for high nickel/silicon carbon batteries in practical applications.For the purpose of reusing the vehicle traction battery after decommissioning,the paper constructs three scenarios by adjusting the intensity of the cycle conditions,simulating the multi-stage recycling of lithium iron phosphate traction batteries,and using CT scanning equipment to detect the internal structure of different stages.The same battery samples were scanned and analyzed at the same time.Taking the backup power condition of the communication base station as the experimental condition,the battery usable life evaluation model under different temperatures and different constant-voltage charging voltages was studied.Through the adiabatic thermal runaway test,the change law of thermal stability with life decay of lithium iron phosphate batteries in different states of health was studied.The experimental results show that:（1）The battery has a long working time in each stage of the experiment after the three-stage strength test under different cycle conditions.In the second stage,the battery can work for 530 cycles,about 2100 h under 0.5C discharge and 480cycles,about 1200 h under 2C discharge.In the third stage,most of the batteries can reach 450 cycles under 0.5C charge and discharge and still hold more than 80%of the initial capacity.It shows that the lithium iron phosphate battery for electric vehicles still has reuse value after decommissioning;（2）According to the battery usable life evaluation model established in the paper under the backup power condition of the communication base station,it is predicted that the usable life is about 5.0 years in the environment of 25℃,and the usable life is about 1.3 years at 55℃.The usable life is about 6.3 years under the constant-voltage charging voltage of 3.60 V,and the usable life is 5.0 years under 3.50 V and 3.65 V;（3）The self-generated heat temperature of the battery gradually decreases with the life decay,indicating that the thermal stability of the sample increases with the health.The change of state shows a gradually worsening trend.This research work verifies the multi-stage recycling capability of lithium iron phosphate batteries for vehicles,and the prediction model and thermal safety provide technical support for the recycling process.The research of this paper provides technical support for EVS-GTR and national mandatory standard GB38031-2020"Safety Requirements for traction batteries for Electric Vehicles". |
PDF Full Text Request